Photosensitivity diseases constitute an important segment of dermatological illness; their effects can be severely disabling and persistent. Since sunlight exposure is inevitable, everyone is at risk; this risk is increased by the wide array of artificial light sources in the environment and the continual introduction of new substances with uncertain phototoxic and photoallergic properties, particularly in industry. Workers are at special risk since they handle materials at higher concentrations and deal with materials not necessarily intended for use on skin. Safe and effective ways to prevent, limit or reverse the pathological effects of light on skin will benefit many patients and potential patients. We will investigate photoallergic skin diseases, beginning with a study of photoallergic contact dermatitis (photo-ACD). The work will be done primarily in the mouse, with subsidiary studies in the rat and will make use of a methodology that we have recently developed for producing photo-ACD in mice. We will 1) study mechanisms, paying particular attention to how light renders immunogeneic a photoallergen applied to the skin, 2) develop a bioassay in the mouse that will identify and grade photocontact allergens of man, and 3) develop an in vitro assay for photo-ACD in the mouse based on the increased synthesis of DNA of sensitized lymphocytes when confronted with photoallergen. The results of this work will assist in the prevention and control of photo-ACD, and will provide a basis for elucidating mechanisms that control other photoallergic diseases.